---
title: "Lux to PPFD Converter by Light Source | Plant Porch"
description: "Convert lux or foot-candles to PPFD (µmol/m²/s) and back for sunlight, white LED, fluorescent, metal halide and sodium grow lights, with the caveats."
url: https://plantporch.me/lux-to-ppfd-converter/
---

Light meters and grow lights

# Lux to PPFD converter

Light meters and phone apps read in lux or foot-candles, which measure light the way our eyes see it. Plants respond to PPFD. Pick the light source to get an approximate conversion either way.

## Why the light source matters

Lux is weighted by the human eye, which is most sensitive to green-yellow light near 555 nm. PPFD counts every photon from 400 to 700 nm equally, including the red and blue that plants use well. So the same lux reading means a different PPFD under the sun, a fluorescent tube or a sodium lamp: sunlight works out at about 54 lux per µmol/m²/s, high-pressure sodium at 82. For white LEDs we use 1,000 lux ≈ 15 µmol/m²/s, from an analysis of many white LED spectra.

A foot-candle is one lumen per square foot, which is 10.764 lux. The gauge shows Missouri Extension's houseplant light levels in foot-candles: low 50–250, medium 250–1,000 and high from 1,000.

Measure at the top of the plants with the sensor held level and clean, as Purdue's guide advises. Virginia Tech notes that a phone app can give a home grower useful information, but a quantum (PAR) sensor reads PPFD directly.

## Sources

- Apogee Instruments, [Conversion – PPFD to Lux](https://www.apogeeinstruments.com/conversion-ppfd-to-lux/): lux per µmol/m²/s for sunlight (54), cool white fluorescent (74), high-pressure sodium (82), double-ended HPS (77), metal halide (71) and ceramic metal halide (65 at 4200 K, 59 at 3100 K), with the definitions of lux and the foot-candle. Its reverse table (0.0185 and so on) agrees within 0.5%.
- Thimijan RW, Heins RD. Photometric, radiometric, and quantum light units of measure: a review of procedures for interconversion. *HortScience* 1983;18(6):818–822. [doi:10.21273/HORTSCI.18.6.818](https://doi.org/10.21273/HORTSCI.18.6.818). The source of the per-lamp factors.
- Sharakshane A. [An easy estimate of the PFDD for a plant illuminated with white LEDs: 1000 lx = 15 µmol/s/m²](https://doi.org/10.1101/289280). bioRxiv 289280, 2018. A preprint, not peer reviewed: the white LED factor.
- Stallknecht E. [Calculating and Using Daily Light Integral (DLI)](https://www.pubs.ext.vt.edu/SPES/spes-720/spes-720.html). Virginia Cooperative Extension SPES-720NP, 2025: foot-candles per µmol/m²/s (sunlight 5.01, HPS 7.62, metal halide 6.60, cool white fluorescent 6.87), quantum sensors and phone apps.
- Torres AP, Lopez RG. [Measuring Daily Light Integral in a Greenhouse](https://www.extension.purdue.edu/extmedia/ho/ho-238-w.pdf). Purdue Extension HO-238-W: keeping light sensors level and clean.
- Trinklein DH. [Lighting Indoor Houseplants](https://extension.missouri.edu/publications/g6515). University of Missouri Extension G6515: low, medium and high light in foot-candles.

## Questions

**What about purple (red and blue) LED grow lights?**

No single factor works for them. Lux follows the eye, which sees deep red and blue poorly, so a lux reading says little about these lights, and none of the sources gives a factor. Use the maker's PPFD chart or a PAR meter.

**Is the result exact?**

No. Each factor is a typical figure for that kind of light; a particular lamp's colour temperature shifts the real value. The two ceramic metal halide lamps, at 65 and 59 lux per µmol/m²/s, show how much colour temperature alone can change it.

**How do I turn PPFD into a daily total?**

Multiply it by the hours the light is on and by 3,600, then divide by 1,000,000. The table under the result does this for 12 to 18 hours, and the [DLI calculator](https://plantporch.me/) compares the total with your plants' target band.